Reactions between biological molecules exhibit an extremely high degree of specificity. It is this specificity that provides a living cell with the ability to carry out thousands of chemical reactions simultaneously in the same "vessel". In general, this specificity arises from the "fit" between two molecules having very complex surface topologies. For example, an antibody binds a molecule displaying an antigen on its surface because the antibody contains a pocket whose shape is the complement of a protruding area on the antigen. This type of specific binding between two molecules forms the basis of numerous biological assays.
For example, nucleic acids are linear polymers in which the linked monomers are chosen from a class of 4 possible sub-units. In addition to being capable of being linked together to form the polymers in question, each unit has a complementary sub-unit to which it can bind electrostatically. In the case of DNA, the polymers are constructed from four bases that are usually denoted by A, T ,G, and C. The bases A and T are complementary to one another, and the bases G and C are complementary to one another. Consider two polymers that are aligned with one another. If the sequences in the polymers are such that an A in one chain is always matched to a T in the other chain and a C in one chain is always matched to a G in the other chain, then the two chains will be bound together by the electrostatic forces. Hence, an immobilized chain can be used to bind the complementary chain. This observation forms the basis of tests that detect the presence of DNA or RNA that is complementary to a known DNA or RNA chain. Such detection forms the basis of a number of medical and/or diagnostic tests.
The methods by which the binding of the mobile reactant to the immobilized component of the system is measured varies with the particular reactants. However, a significant fraction of all of the tests involve the measurement of a fluorescent dye that is associated with either the bound or mobile reactant. The dye may be attached to the reactant from the beginning of the process or it may be added through various chemical steps after the mobile and immobilized reactants have been brought into contact with one another.
Systems for medical diagnosis often involve a bank of tests in which each test involves the measurement of the binding of one mobile component to a corresponding immobilized component. To provide inexpensive test kits, systems involving a matrix of immobilized spots have been suggested. Each spot includes the immobilized component of a two component test such as described above. The fluid to be tested is typically brought into contact with the matrix. After chemical processing, the amount of fluorescence associated with each of the spots in the matrix is measured.
The matrix is typically constructed by dispensing small quantities of the immobilized component onto a substrate such as glass or filter paper. In general, prior art assays utilizing such matrices require that the matrix remain wet from the point in the process at which the components are dispensed through the detection of the fluorescence. This requirement leads to a number of problems when these assay are applied in medical diagnosis. First, the buffer solutions utilized in the processing may contain contaminants that have fluorescent emission bands sufficiently close to those of the fluorescent compound of interest that the stray fluorescence gives rise to errors in the assay. The amount of interference depends on the amount of buffer needed in the particular system. If the ratio of buffer solution to bound fluorescent compound is high, even a small degree of contamination of the buffer solution can generate unacceptable errors.
A second problem with wet assay plates relates to the transportation and storage thereof In medical diagnostic applications, it is anticipated that the assay plates will be prepared by a commercial supplier and shipped to the diagnostic laboratory. The plates would then be stored at the laboratory until needed. The need to provide leak-proof packaging significantly increases the cost of these assay plates.
In addition, the short storage life of wet assay plates at room temperature places additional constraints on the storage and transportation of these assay plates. The biological macromolecules on which these assays are based are easily attacked by various enzymes. These enzymes often appear as contaminants in the various buffer solutions used in preparing and storing the wet assay plates. As a result, the assay plates must be refrigerated to increase their shelf life. The cost of refrigerated storage and transportation significantly increases the cost of assay systems based on wet assay plates. Furthermore, even with refrigerated storage, the useful shelf life of wet assay plates results in significant increases in costs due to the need to discard old assay plates before they are actually used.
A third problem with wet assay plates is the need to read the results of the assay shortly after the chemical processing of the plates. This restricts the reading and interpretation of the results to the laboratory that processed the patient samples with the plates. While this restriction is not very significant in metropolitan settings, it can be a significant problem in rural settings In which the volume of tests is too low to justify the cost of the equipment and personnel needed to read and interpret the assays. As a result, the patient samples are typically sent to a central laboratory for reading and processing. The need for refrigerated transportation of the samples and the inherent time delays in receiving the results of the tests make this solution to the problem less than optimal.
Finally, prior art wet assay plate systems do not provide a means for archiving the assay plates for later examination since the catabolic enzymes described above will destroy the underlying macromolecules even if the assay plates are stored in a refrigerated environment. An assay plate that provided a cost effective archival storage mechanism which would allow the plate to be read again some time after its original processing would be highly desirable both from a research and a legal point of view. Researchers often wish to examine samples from a large population. The samples in question can often be taken from routine assays if archival storage of the routine samples is available. Unfortunately, present archival storage requires storage of the original samples at liquid nitrogen temperatures. The costs inherent in this approach limit the archiving of samples to special studies.
In legal settings, the ability to re-examine tests performed months or years earlier would be of significant benefit in determining the validity of the earlier performed tests. There are many situations in which the validity of such tests determines the outcome in a legal proceeding.
Broadly, it is the object of the present invention to provide an improved assay plate system for performing assays based on the binding of biological macromolecules.
It is a further object of the present invention to provide an assay plate system in which the assay plates do not need to be shipped or stored in a wet state.
It is a still further object of the present invention to provide an assay plate system in which the assay plates may be read and archived in a dry state.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.